Spectroscopy is a common technique for measuring the concentration of organic and some inorganic constituents of a solution. The theoretical basis of this technique is the Beer-Lambert law, which states that the concentration ci of an absorbent in solution can be determined by the intensity of light transmitted through the solution, knowing the pathlength dλ, the intensity of the incident light I0,λ, and the extinction coefficient εi,λ at a particular wavelength λ.
In generalized form, the Beer-Lambert law is expressed as:
                              I          λ                =                              I                          0              ,              λ                                ⁢                      e                                          -                                  d                  λ                                            ·                              μ                                  a                  ,                  λ                                                                                        (        1        )                                          μ                      a            ,            λ                          =                              ∑                          i              =              1                        n                    ⁢                                    ɛ                              i                ,                λ                                      ·                          c              i                                                          (        2        )            
where μα,λ is the bulk absorption coefficient and represents the probability of absorption per unit length. The minimum number of discrete wavelengths that are required to solve equations 1 and 2 is the number of significant absorbers that are present in the solution.
A practical application of this technique is pulse oximetry, which utilizes a noninvasive sensor to measure oxygen saturation and pulse rate, among other physiological parameters. Pulse oximetry relies on a sensor attached externally to the patient to output signals indicative of various physiological parameters, such as a patient's blood constituents and/or analytes, including for example a percent value for arterial oxygen saturation, among other physiological parameters. The sensor has an emitter that transmits optical radiation of one or more wavelengths into a tissue site and a detector that responds to the intensity of the optical radiation after absorption by pulsatile arterial blood flowing within the tissue site. Based upon this response, a processor determines the relative concentrations of oxygenated hemoglobin (HbO2) and deoxygenated hemoglobin (Hb) in the blood so as to derive oxygen saturation, which can provide early detection of potentially hazardous decreases in a patient's oxygen supply.
A pulse oximetry system generally includes a patient monitor, a communications medium such as a cable, and/or a physiological sensor having one or more light emitters and a detector, such as one or more light-emitting diodes (LEDs) and a photodetector. The sensor is attached to a tissue site, such as a finger, toe, earlobe, nose, hand, foot, or other site having pulsatile blood flow which can be penetrated by light from the one or more emitters. The detector is responsive to the emitted light after attenuation or reflection by pulsatile blood flowing in the tissue site. The detector outputs a detector signal to the monitor over the communication medium. The monitor processes the signal to provide a numerical readout of physiological parameters such as oxygen saturation (SpO2) and/or pulse rate. A pulse oximetry sensor is described in U.S. Pat. No. 6,088,607 entitled Low Noise Optical Probe; pulse oximetry signal processing is described in U.S. Pat. Nos. 6,650,917 and 6,699,194 entitled Signal Processing Apparatus and Signal Processing Apparatus and Method, respectively; a pulse oximeter monitor is described in U.S. Pat. No. 6,584,336 entitled Universal/Upgrading Pulse Oximeter; all of which are assigned to Masimo Corporation, Irvine, Calif., and each is incorporated by reference herein in its entirety.
There are many sources of measurement error introduced to pulse oximetry systems. Some such sources of error include the pulse oximetry system's electronic components, including emitters and detectors, as well as chemical and structural physiological differences between patients. Another source of measurement error is the effect of multiple scattering of photons as the photons pass through the patient's tissue (arterial blood) and arrive at the sensor's light detector.